JP2011117009A - Steel having excellent rolling fatigue life - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel whose rolling fatigue life can be further improved without deteriorating its manufacturability. <P>SOLUTION: The steel contains each 0.65 to 1.30% C, 0.05 to 1.00% Si, 0.1 to 2.00% Mn, &le;0.050% (not including 0%) P, &le;0.050% (not including 0%) S, 0.15 to 2.00% Cr, 0.010 to 0.100% Al, &le;0.025% (not including 0%) N, &le;0.015% (not including 0%) Ti and &le;0.0025% (not including 0%) O, and the balance iron with inevitable impurities, wherein the average circle-equivalent diameter of Al based nitrogen compounds dispersed into the steel is 25 to 200 nm, and further, the piece density of the Al based nitrogen compounds with a circle-equivalent diameter of 25 to 200 nm is 1.1 to 6.0 pieces/&mu;m<SP>2</SP>. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a steel material applied to bearing parts and machine structural parts used in automobiles, various industrial machines, etc., and in particular, a steel material that exhibits excellent rolling fatigue life when used as the various members. It is about.

  Parts such as bearings and crankshafts are important parts that support the rotating parts and sliding parts of machinery, and the contact surface pressure is considerably high and the external force may fluctuate. In many cases, excellent durability is required for the steel material.

  In recent years, these requirements have become stricter year by year as the performance and weight of machinery have been improved. In order to improve the durability of shaft parts, it is important to improve the technology related to lubricity, but it is particularly important that the steel material has excellent rolling fatigue characteristics.

  As steel materials used for bearings, high-carbon chromium bearing steels such as SUJ2 as defined in JIS G 4805 (1999) have been used as bearing materials that have been used in various fields such as automobiles and various industrial machines. Has been. However, since bearings are used in harsh environments such as inner and outer rings and rolling elements such as ball bearings and roller bearings with extremely high contact surface pressure, fatigue failure is likely to occur due to very fine defects (inclusions, etc.). There is a problem. In order to solve this problem, attempts have been made to improve the steel for bearings in order to increase the rolling fatigue life itself and reduce the number of maintenance operations.

  For example, in Patent Document 1, the content of Ti and Al in the bearing material is specified, and the amount of fine Ti carbide, Ti carbonitride, Al nitride, etc. is determined by performing heat treatment after spheroidizing annealing. It has been proposed to improve the rolling fatigue life by controlling and refining the prior austenite crystal grains (former γ crystal grains).

  However, in the above technique, the Ti content is very high at 0.26% or more, and there is a problem that not only the cost is increased but also the workability is lowered. In addition, coarse TiN is likely to be generated during casting, and the fatigue life may vary due to the formation of this precipitate. On the other hand, the Al content is also 0.11% or more, and the Al-based nitrogen compound produced during casting and rolling may cause cracks and scratches, resulting in poor productivity.

Japanese Patent No. 3591236

  This invention is made | formed in view of such a situation, The objective is to provide the steel materials which can further improve a rolling fatigue life, without deteriorating manufacturability.

The steel materials according to the present invention that have achieved the above-mentioned object are: C: 0.65 to 1.30% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 1.00%, Mn: 0 0.1 to 2.00%, P: 0.050% or less (not including 0%), S: 0.050% or less (not including 0%), Cr: 0.15 to 2.00%, Al : 0.010 to 0.100%, N: 0.025% or less (not including 0%), Ti: 0.015% or less (not including 0%), and O: 0.0025% or less (0% Each of which is composed of iron and inevitable impurities, and the Al-based nitrogen compound dispersed in the steel has an average equivalent circle diameter of 25 to 200 nm and an equivalent circle diameter of 25 to 200 nm. the number density of the compound is 1.1 pieces / [mu] m ² or more, have a summary to the point at 6.0 pieces / [mu] m ² or less Is shall.

  The above “equivalent circle diameter” refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of the Al-based nitrogen compound, and is a transmission electron microscope (TEM) or scanning. Of an Al-based nitrogen compound observed on the observation surface of a scanning electron microscope (SEM). In addition, the Al-based nitrogen compounds targeted in the present invention include not only AlN, but also those containing a part of elements such as Mn, Cr, S and Si (total content up to about 30%). It is.

  In the steel material of the present invention, the prior austenite preferably has an average grain size number of 11.5 or less, and by satisfying these requirements, the rolling fatigue life is further improved.

  Further, in the steel material of the present invention, if necessary, as another element, (a) Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%) And Mo: one or more selected from the group consisting of 0.25% or less (not including 0%), (b) Nb: 0.5% or less (not including 0%), V: 0.5% 1 or more selected from the group consisting of the following (excluding 0%) and B: 0.005% or less (not including 0%), (c) Ca: 0.05% or less (not including 0%) ), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), and Zr: 0 .One or more selected from the group consisting of 2% or less (not including 0%), (d) Pb: 0.5% or less (not including 0%), Bi It is also useful to contain one or more selected from the group consisting of 0.5% or less (excluding 0%) and Te: 0.1% or less (not including 0%), etc. The properties of the steel material are further improved depending on the components to be added.

  According to the present invention, a steel material that further improves the rolling fatigue life by appropriately adjusting the chemical component composition and appropriately dispersing an appropriately sized Al-based nitrogen compound in the steel material has good manufacturability. Therefore, when such a steel material is applied to a bearing or the like, an excellent rolling fatigue life can be exhibited even when used in a harsh environment.

Is a graph showing the relationship between the number density and fatigue life L ₁₀ of the Al-based nitrogen compounds. It is a graph which shows the number density and magnitude | size relationship of Al type nitrogen compound. It is a graph showing the relationship between fatigue life L ₁₀ the old γ grain size number. It is a graph which shows the relationship between a primary cooling rate and the magnitude | size of an Al type nitrogen compound.

  The present inventors have studied from various angles with the aim of realizing a steel material excellent in rolling fatigue characteristics (rolling fatigue life) without deteriorating manufacturability. And in order to improve the rolling fatigue life of steel materials, the knowledge that it was effective to satisfy the following requirements (A) to (D) was obtained.

(A) Although the Al content is small, a large amount of fine Al-based nitrogen compound is dispersed, and by the dispersion strengthening, crack generation / propagation can be suppressed, and a good rolling fatigue life can be obtained.
(B) In order to suppress cracking during casting and rolling, it is necessary to regulate the amount (number density) and size of the Al-based nitrogen compound,
(C) In order to achieve the degree of dispersion (number density) in fine Al-based nitrogen compounds, it is important to strictly control the content of Al and N in the steel, and in the manufacturing process of steel It is useful to increase the cooling rate after removing the temperature range of 850 to 650 ° C., which is the precipitation temperature range of the Al-based nitrogen compound after hot rolling,
(D) If the crystal grains of the prior austenite (former γ) are too fine, the hardenability is lowered, so that an incompletely quenched phase is likely to be generated, and the rolling fatigue life tends to be reduced.

Based on the above findings, the present inventors have further conducted intensive studies to improve the rolling fatigue life of steel materials. As a result, the Al and N contents in the steel material are strictly defined, the production conditions are controlled, and the average equivalent circle diameter of the Al-based nitrogen compound dispersed in the steel after quenching and tempering is 25 to 200 nm. If the number density of Al-based nitrogen compounds having an equivalent circle diameter of 25 to 200 nm is 1.1 / μm ² or more and 6.0 / μm ² or less, the rolling fatigue life of the steel material is remarkably improved. The present invention has been completed by finding out what can be done.

  In the steel material of the present invention, it is an important requirement to appropriately control the number density of Al-based nitrogen compounds having an equivalent circle diameter of 25 to 200 m, but the dispersion strengthening suppresses the generation and propagation of cracks, and is good A long rolling fatigue life is achieved. For that purpose, it is necessary to appropriately control the size of the Al-based nitrogen compound, and when this size (average circle equivalent diameter) is smaller than 25 nm or larger than 200 nm, the effect of dispersion strengthening is exhibited. Can not be. The size of the Al-based nitrogen compound is preferably 40 nm or more (more preferably 50 nm or more), preferably 150 nm or less (more preferably 125 nm or less).

If the number density of Al-based nitrogen compounds having an equivalent circle diameter of 25 to 200 m is less than 1.1 / μm ² , the rolling fatigue life improving effect due to dispersion strengthening cannot be effectively exhibited, and 6.0 / μm ² is reduced. If exceeding, the crystal grains become coarse, an incompletely quenched phase (for example, fine pearlite or bainite phase) is generated, and the rolling fatigue life is reduced. The number density of the Al-based nitrogen compound is preferably 1.5 / μm ² or more (more preferably 2.0 / μm ² or more), preferably 5.0 / μm ² or less (more preferably 4). 0.0 pieces / μm ² or less).

  In the steel material of the present invention, it is effective to control crystal grains of prior austenite (former γ). The larger the grain size number of the former γ (the smaller the crystal grain), the better the hardness and the crack propagation characteristics, but if the grain size number becomes too large (if the crystal grain becomes too small) ), Hardenability is reduced, and an incompletely hardened phase is likely to be formed. On the contrary, the rolling fatigue life is reduced. Therefore, the grain size number of the old γ is preferably 11.5 or less, more preferably 11.0 or less (more preferably 10.5 or less).

  In the steel material of the present invention, the chemical component composition (C, Si, Mn, P, S, Cr, Al, N, Ti, O) including the above-described Al and N contents needs to be appropriately adjusted. However, the reasons for limiting the ranges of these components are as follows.

[C :: 0.65 to 1.30%]
C is an essential element for increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. In order to exert such an effect, C must be contained in an amount of 0.65% or more, preferably 0.8% or more (more preferably 0.95% or more). However, if the C content is excessively large, giant carbides are likely to be generated and adversely affect the rolling fatigue characteristics. Therefore, the C content is 1.30% or less, preferably 1.2% or less ( More preferably, it should be suppressed to 1.1% or less.

[Si: 0.05-1.00%]
Si is an element useful for improving the solid solution strengthening and hardenability of the matrix. In order to exert such effects, it is necessary to contain Si by 0.05% or more, preferably 0.1% or more (more preferably 0.15% or more). However, since the workability and machinability are remarkably lowered when the Si content is excessively large, the Si content is 1.00% or less, preferably 0.9% or less (more preferably 0.8% or less). Should be suppressed.

[Mn: 0.1 to 2.00%]
Mn is an element useful for improving the solid solution strengthening and hardenability of the matrix. In order to exert such an effect, it is necessary to contain Mn in an amount of 0.1% or more, preferably 0.15% or more (more preferably 0.2% or more). However, if the Mn content becomes too large, the workability and machinability are remarkably lowered, so the Mn content is 2.00% or less, preferably 1.6% or less (more preferably 1.2% or less). Should be suppressed.

[P: 0.050% or less (excluding 0%)]
P is an element inevitably contained as an impurity, but it is desirable to reduce it as much as possible because it segregates at the grain boundary and lowers the workability, but extremely reducing causes an increase in steelmaking cost. . For these reasons, the P content is set to 0.050% or less. Preferably, it is good to reduce to 0.04% or less (more preferably 0.03% or less).

[S: 0.050% or less (excluding 0%)]
S is an element that is inevitably contained as an impurity, but precipitates as MnS, and it is desirable to reduce it as much as possible in order to reduce the rolling fatigue life. However, extreme reduction leads to an increase in steelmaking costs. Become. For these reasons, the S content is set to 0.050% or less. Preferably, it is good to reduce to 0.04% or less (more preferably 0.03% or less).

[Cr: 0.15-2.00%]
Cr is an element that combines with C to form carbides, imparts wear resistance, and contributes to improving hardenability. In order to exert such an effect, the Cr content needs to be 0.15% or more. Preferably it is 0.5% or more (more preferably 0.9% or more). However, when the Cr content is excessive, coarse carbides are generated and the rolling fatigue life is decreased. Accordingly, the Cr content is 2.00% or less. Preferably it is 1.8% or less (more preferably 1.6% or less).

[Al: 0.010-0.100%]
Al is an element that plays an important role in the steel material of the present invention, and when it is combined with N, it is finely dispersed in the steel as an Al-based nitrogen compound, and is important for improving the rolling fatigue life of the steel material. It is an element. In order to produce a fine Al-based nitrogen compound, it is necessary to contain at least 0.010% or more. However, if the Al content becomes excessive and exceeds 0.100%, the size and number of Al-based nitrogen compounds that are precipitated increase, and cracks and scratches are likely to occur during casting and rolling. Further, since the crystal grains become too fine, the hardenability is lowered, cannot be applied to a large part, and the rolling fatigue life is lowered. The preferable lower limit of the Al content is 0.013% (more preferably 0.015% or more), and the preferable upper limit is 0.08% (more preferably 0.05% or less).

[N: 0.025% or less (excluding 0%)]
N, like Al, is an element that plays an important role in the steel material of the present invention, and is an important element for exerting an effect of improving the rolling fatigue life due to fine dispersion of an Al-based nitrogen compound. However, when the N content becomes excessive and exceeds 0.025%, the size and number density of the Al-based nitrogen compound to be precipitated increase, and cracks are likely to occur during casting and rolling. Further, since the crystal grains become too fine, the hardenability is lowered, cannot be applied to a large part, and the rolling fatigue life is lowered. The lower limit of the N content is not particularly limited as long as a predetermined amount of Al-based nitrogen compound can be precipitated, and the cooling rate after rolling and the amount of elements (Ti, V, Nb, B, Zr, Te, etc.) that bind to N are reduced. And what is necessary is just to set suitably according to Al content. For example, when the N content is 0.0035% or more, a predetermined amount of an Al-based nitrogen compound can be precipitated. In addition, the minimum with preferable N content is 0.004% (more preferably 0.006% or more), and a preferable upper limit is 0.020% (more preferably 0.022% or less).

[Ti: 0.015% or less (excluding 0%)]
Ti combines with N in steel to produce TiN, which not only adversely affects rolling fatigue properties but also harms cold workability and hot workability, and it is desirable to reduce it as much as possible. However, an extreme reduction leads to an increase in steelmaking costs. For these reasons, the Ti content needs to be 0.015% or less. In addition, the upper limit with preferable Ti content is 0.01% (more preferably 0.005% or less).

[O: 0.0025% or less (excluding 0%)]
O has a large effect on the form of impurities in steel and forms inclusions such as Al ₂ O ₃ and SiO ₂ that adversely affect rolling fatigue characteristics. Doing so will increase the steelmaking cost. For these reasons, the O content needs to be 0.0025% or less. In addition, the upper limit with preferable O content is 0.002% (more preferably 0.0015% or less).

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities. As the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. In order to increase the rolling fatigue life, the following elements can be positively contained within a specified range.

[Selected from the group consisting of Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (not including 0%) One or more types]
Cu, Ni, and Mo are all elements that act as a hardenability improving element of the parent phase and contribute to improving rolling fatigue characteristics by increasing hardness. All of these effects are effectively exhibited by containing 0.03% or more. However, if any content exceeds 0.25%, the workability deteriorates.

[Nb: 0.5% or less (not including 0%), V: 0.5% or less (not including 0%) and B: 0.005% or less (not including 0%) One or more types]
Nb, V, and B are all effective elements for bonding with N to form a nitrogen compound to adjust the grain size and improve the rolling fatigue life. However, when Nb or V exceeds 0.5% and B exceeds 0.005%, the crystal grains become finer, and an incompletely quenched phase tends to be generated. A more preferable upper limit is 0.3% (more preferably 0.1% or less) for Nb and V, and 0.003% (more preferably 0.001% or less) for B.

[Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.0. 02% or less (not including 0%) and Zr: one or more selected from the group consisting of 0.2% or less (not including 0%)]
Ca, REM (rare earth element), Mg, Li, and Zr are all elements that spheroidize oxide inclusions and contribute to improving the rolling fatigue life. These effects are effectively exhibited by containing 0.0005% or more in Ca or REM and 0.0001% or more in Mg, Li or Zr. However, even if it is contained excessively, the effect is saturated, and an effect commensurate with the content cannot be expected, which is uneconomical. A more preferable upper limit is 0.03% (more preferably 0.01% or less) for Ca or REM, 0.01% (more preferably 0.005% or less) for Mg or Li, and 0.15 for Zr. % (More preferably 0.10% or less).

[Pb: selected from the group consisting of 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%), and Te: 0.1% or less (not including 0%) One or more types]
Pb, Bi, and Te are all machinability improving elements. These effects are effectively exhibited by containing 0.01% or more of Pb and Bi and 0.0001% or more of Te. However, if the content of Pb and Bi exceeds 0.5% or the content of Te exceeds 0.1%, production problems such as generation of rolling flaws occur. A more preferable upper limit is 0.3% (more preferably 0.2% or less) for Pb and Bi, and 0.075% (more preferably 0.05% or less) for Te.

In the steel material of the present invention, in order to disperse the fine Al-based nitrogen compound in the steel after quenching and tempering, a slab satisfying the above component composition is used in the steel material production process, and the cooling rate after rolling is controlled. This is very important. The Al-based nitrogen compound that precipitates in the cooling process after rolling remains in the same state even after the subsequent spheroidizing annealing, parts processing, quenching / tempering process. Therefore, the temperature range from 850 to 650 ° C., which is the precipitation temperature range of the Al-based nitrogen compound, is set to a range of 0.10 to 0.90 ° C./second in terms of primary cooling rate (average cooling rate), from less than 650 ° C. to room temperature. By cooling the secondary cooling rate (average cooling rate) to (25 ° C.) at 1 ° C./second or more, the average equivalent circle diameter of the Al-based nitrogen compound is set to 25 to 200 nm even in the steel after quenching and tempering. The Al-based nitrogen compound having an equivalent circle diameter of 25 to 200 nm can be dispersed in a range of 1.1 / μm ² or more and 6.0 / μm ² or less.

When the primary cooling rate is less than 0.10 ° C./second, the Al-based nitrogen compound becomes coarse, and when it exceeds 0.90 ° C./second, the average equivalent circle diameter of the Al-based nitrogen compound is less than 25 nm, The number density of the predetermined size is less than 1.1 / μm ² , and the desired size and number cannot be obtained. Further, by setting the secondary cooling rate below 650 ° C. to 1 ° C./more, it is possible to suppress the coarsening of the Al-based nitrogen compound and to control its size.

  The steel material of the present invention is manufactured into bearing parts and the like after being quenched and tempered into a predetermined part shape, but the shape of the steel material stage is either linear or rod-like that can be applied to such production. The size can also be appropriately determined according to the final product.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can be adapted to the purpose described above and below. Of course, they are all possible and are included in the technical scope of the present invention.

  The steel materials (test Nos. 1 to 51) having various chemical composition compositions shown in Tables 1 and 2 below were heated to 1100 to 1300 ° C in a heating furnace or a soaking furnace, and then subjected to split rolling at 900 to 1200 ° C. Then, after heating to 900-1100 degreeC, it rolled (including the forging imitating rolling), and produced the round bar material of 70 mm in diameter. After rolling, the temperature is cooled from 850 to 650 ° C. at various average cooling rates (Tables 3 and 4 below), and from 650 ° C. to room temperature (25 ° C.) at an average cooling rate of 1 ° C./second. Rolled material or forged material was obtained.

  The rolled material or forged material was subjected to spheroidizing annealing at 795 ° C. (holding time: 6 hours), and then subjected to skin cutting. Thereafter, a disk having a diameter of 60 mm and a thickness of 5 mm was cut out and subjected to oil quenching after heating at 840 ° C. for 30 minutes and tempering at 160 ° C. for 120 minutes. Finally, finish polishing was performed to prepare a test piece having a surface roughness Ra (arithmetic average roughness) of 0.04 μm or less.

  Using the test piece obtained above, the number and size of Al-based nitrogen compounds and the crystal grains (grain size number) of prior austenite (former γ) were measured under the following conditions, and the fatigue life and presence or absence of cracks Evaluated.

[Measurement of number and size of Al-based nitrogen compounds]
As a method for confirming the dispersion state of the Al-based nitrogen compound, the test piece after the heat treatment was cut, this section was polished, carbon was then deposited on the surface, and FE-TEM (field emission transmission electron microscope) was used. Replica observation was performed. At this time, components of an Al-based nitrogen compound containing Al and N were specified by TEM EDX (energy dispersive X-ray detector), and the field of view was observed at a magnification of 30000 times. At this time, 1 field of view was set to 16.8 μm ^2, and arbitrary 3 fields of view were observed (total 50.4 μm ² ), and particle analysis software [“Particle Analysis III for Windows. Version 3.00 SUMITOMO METAL TECHNOLOGY” (trade name)] Was used to determine the size (average circle equivalent diameter) and the number of Al-based nitrogen compounds having an equivalent circle diameter of 25 to 200 nm (number is converted per μm ² : number density).

[Measurement of crystal grains (crystal grain size number) of prior austenite (former γ)]
After cutting the heat-treated test piece and polishing the cross section, the former austenite grain boundary corrosion was performed, and four 150 μm depth positions were photographed from the surface layer, and in accordance with JIS G 0551 (method based on standard diagram) ) Old austenite particle size measurement was carried out.

[Measurement of fatigue life]
Using a thrust type rolling fatigue tester, rolling fatigue tests were performed 16 times for each steel material (test piece) under the conditions of repetition rate: 1500 rpm, surface pressure: 5.3 GPa, number of cancellations: 2 × 10 ⁸ times. The fatigue life L ₁₀ (the number of stress repetitions until fatigue failure at a cumulative failure probability of 10% obtained by plotting on the wiper probability paper) was evaluated. At this time, the fatigue life L ₁₀ (L ₁₀ life) was 1.0 × 10 ⁷ times or more as an acceptance criterion.

[Evaluation of cracks]
The surface of the sample after rolling and forging was cut, and the surface was visually observed. When a scratch of 3 mm or more was observed, it was determined that there was a crack.

  These results are shown in Tables 3 and 4 below together with production conditions (average cooling rate up to 850 to 650 ° C., presence or absence of secondary cooling).

  From these results, it can be considered as follows. That is, test no. 3-5, 8, 10, 11, 14, 16-22, 27-32 are the requirements (chemical composition, size and number of Al-based nitrogen compounds) or preferred requirements (old γ) defined in the present invention. It can be seen that excellent rolling fatigue life is achieved without any cracks.

  In contrast, test no. 1, 2, 6, 7, 9, 12, 13, 15, 23 to 26, 33 to 51, any of the requirements defined in the present invention is outside, so all have low rolling fatigue life It has become.

  Test No. For 1, 6, 15, 23, 26, 33, 35, 37, and 38, the cooling conditions after rolling are not appropriate, so the size of the Al-based nitrogen compound is too large (among these) , Test Nos. 23, 26, 33, 37, and 38 have the old γ grain size number also deviated), and all have a low rolling fatigue life.

  Test No. Tests Nos. 2, 7, 9, 24, and 25 have a high cooling rate. In No. 40, since the Ti content increases and TiN is formed, the number of Al-based nitrogen compounds is insufficient. In No. 34, since the Al content is larger than the range specified in the present invention, the number density and size of the Al-based nitrogen compound are excessive, and both have rolling fatigue life. It is low.

  Test No. In Nos. 12 and 13, the number density of the number of Al-based nitrogen compounds is excessive, and the former γ grain size number, which is a preferable requirement, is outside the range defined in the present invention. The dynamic fatigue life is low.

  Test No. 36 to 39 and 41 to 51 are those that deviate from the chemical composition defined in the present invention (test Nos. 37 and 38 also deviate from the above requirements), and both have a low rolling fatigue life. .

Based on these data, FIG. 1 shows the relationship between the number density of an Al-based nitrogen compound (Al-based nitrogen compound having an equivalent circle diameter of 25 to 200) and the fatigue life L ₁₀ . The relationship of (average circle equivalent diameter) is shown in FIG. 2 (the plots satisfying the range defined by the present invention in terms of chemical components), respectively, but the number density and size of Al-based nitrogen compounds are appropriately controlled. It can be seen that an excellent fatigue life L ₁₀ (rolling fatigue life) is achieved.

The relationship between the old γ grain size number and the fatigue life L ₁₀ is shown in FIG. 3. Setting the old γ grain size number to an appropriate range is necessary to achieve an excellent fatigue life L ₁₀ (rolling fatigue life). It turns out that it is an effective means. FIG. 4 shows the relationship between the primary cooling rate (average cooling rate) and the size of the Al-based nitrogen compound (the average equivalent circle diameter of the Al-based nitrogen compound), but adjusting the primary cooling rate to an appropriate range It can be seen that this is effective in controlling the size of the Al-based nitrogen compound.

Claims (6)

C: 0.65-1.30% (meaning of mass%, the same applies hereinafter), Si: 0.05-1.00%, Mn: 0.1-2.00%, P: 0.050% or less ( 0% not included), S: 0.050% or less (not including 0%), Cr: 0.15 to 2.00%, Al: 0.010 to 0.100%, N: 0.025% The following (not including 0%), Ti: 0.015% or less (not including 0%) and O: 0.0025% or less (not including 0%) are included, and the balance is composed of iron and inevitable impurities. 5. The average equivalent circle diameter of the Al-based nitrogen compound dispersed in the steel is 25 to 200 nm, and the number density of Al-based nitrogen compounds having an equivalent circle diameter of 25 to 200 nm is 1.1 / μm ² or more. A steel material excellent in rolling fatigue life characterized by being 0 piece / μm ² or less.   The steel material excellent in rolling fatigue life according to claim 1, wherein the prior austenite has an average grain size number of 11.5 or less.   Further, as other elements, Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%) and Mo: 0.25% or less (not including 0%) The steel material excellent in rolling fatigue life of Claim 1 or 2 containing 1 or more types selected from the group which consists of.   Further, as other elements, Nb: 0.5% or less (not including 0%), V: 0.5% or less (not including 0%), and B: 0.005% or less (not including 0%) The steel material excellent in rolling fatigue life in any one of Claims 1-3 containing 1 or more types selected from the group which consists of.   Further, as other elements, Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%) , Li: not more than 0.02% (not including 0%) and Zr: not less than 0.2% (not including 0%), including at least one selected from the group consisting of Steel material with excellent rolling fatigue life as described in 1.   Further, as other elements, Pb: 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%), and Te: 0.1% or less (not including 0%) The steel material excellent in rolling fatigue life in any one of Claims 1-5 containing 1 or more types selected from the group which consists of.

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